Direct positive emulsion



atented Dec. 19, 1939 UNITED. STATES PATENT OFFICE DIRECT POSITIVE EMULSION Application January 22, 1938, Serial No. 186,399

14 Claim.

This invention relates to photographic emulsions and particularly to a method of making emulsions which produce positive images directly on exposure and development. It is well known that positive images may be produced directly with many photographic emulsions by merely over-exposing them and thereby reaching the solarization region. Positive images produced in this way do not have high contrast and for this reason are unsatisfactory for commercial use. It has been proposed at various times to produce direct positive emulsions which have greater con-' trast. Most of these methods are based on the use of agents or methods of procedure which induces fogging of the emulsion prior to use, as by adding fogging agents, heating, exposing to light rays, or combinations of these methods prior to coating the emulsion on the support.

I have found that direct positive emulsions of 0 high contrast may be produced in a manner not previously described and that the emulsions produced according to my process have distinct advantages over the emulsions hitherto known. My

of the emulsions, as measured by the exposure 30 necessary to reach the minimum of the reversal curve, is in some cases greater than that obtainable by other methods.

My method of producing direct positive emulsions consists essentially in adding the proper 35 amount of a non-acid optical sensitizing dye to the emulsion. A further feature of my invention consists in adding small amounts of certain reducing agents, such as photographic developers, along with the optical sensitizer for the purpose 40 of increasing the maximum density obtainable.

I have discovered that if there is added to a photographic emulsion amounts of a non-acid optical sensitizing dye which are greater than the optimum value for negative sensitivity, the

45 emulsion suddenly exhibits reversal characteristics on prolonged exposure. The emulsions so prepared give little or no fog and the minimum densities on the reversal curves are low. The

with the sensitizing dye extremely small amounts 55 of certain reducing agents, principally photographic developers, the maximum densities may be raised to values as great as 3.0 or higher with no increase in the minimum densities. The introduction of these developing agents produces dye added thereto and one containing the opti- 10 mum amount of sensitizing dye for negative purposes, and Fig. 2 is a graph showing the H. and D. curves for a direct positive emulsion made according to my invention containing increased amounts of an optical sensitizing dye and a simi- 15 lar emulsion containing, in addition to the sen-I sitizing dye, a small amount of a reducing agent.

My-method of producing direct positive emulsions consists in adding a non-acid optical sensitizing dye necessary to produce the desired reversal effect varying with the dye and the emulsion'. As increasing amounts of sensitizing dye are added to an emulsion the sensitivity increases up to a certain value beyond which increasing dye concentration results in photographic materials of poor quality for negative purposes. When this region is reached and sensitizing dyes in the amount of about 50% above the optimum value for negative sensitivity are added to the emulsion, highly desirable reversal emulsions are produced. Thepreferred amount of sensitizing dye is that of about 50% in excess of the optimum value for negative sensitivity. Up to four times or more of the optimum amount for negative sensitivity may be usedbut increasing amounts tend to lower the maximum density obtainable.

I have found that many non-acid optical sensitizers of the common classes produce reversal emulsions according to my invention. These dyes 40 may be, for example, of the cyanine, carbocyanine, thiocarbocya'nine, merocyanine, hemicyanine, hemioxonol an'd styryl classes. The merocyanine optical sensitizing dyes are described in Brooker United States Patent No. 2,078,233, April 27, 1937, the hemicyanine optical sensitizing dyes are described in White and Keyes United States application, Serial No. 138,634, filed April 23, 1937, and the hemioxonol optical sensitizing dyes are described in Keyes United States application, Serial No. 146,425 filed June 4, 1937.

Acid sensitizing dyes, such as erthyrosln do not in general produce reversal.

The optimum amount of optical sensitizing dye for use in photographic emulsions is a well rec- .conditions.

ognized factor in the sensitizing art. The optimum amount of sensitizing dye varies with the dye itself and with the emulsion in which it is used but in every case the optimum amount can be determined quite definitely for a given set of For example, see Brooker United States Patent No. 1,846,302, February 23, 1932, page 4, lines 51 to 69. The optimum or maximum sensitizing effect of any-sensitizing dye may be determined by simple measurement of the emulsion speed obtained and is, therefore, a definite and critical factor. In referring to the use of an optical sensitizer in the present invention in amounts of 50% greater than that necessary for optimum negative sensitizing, I have defined the amount of sensitizer by reference to recognized and easily determined constants.

As I have stated above, the maximum density of the emulsions produced according to my invention is raised by adding to the emulsion containing the optical sensitizer, certain reducing agents, such as photographic developers. The emulsion is not treated in any way which would produce fogging after addition of these compounds and I have no intention of obtaining a fogged emulsion according to my process. The developing agents are not essential in my invention but are useful in bringing about a greater degree of contrast by raising the maximum density produced without afiecting the minimum density. Any of the well known developing agents may be used, such as the para-phenylenediamines, the para aminophenols, hydroquinone, pyrogallol, parahydroxylphenylglycine or any other suitable developing compound. These developing agents are used in the amount of from 0 to 200 milligrams per 1000 grams of silver halide in the emulsion. The preferred amount is about 50 milligrams per 1000 grams of silver bromide in the emulsion.

I have referred to "optical sensitizing dyes, in

the present description of my invention. The

distinction between chemical sensitizers and optical sensitizers will be understood by those skilled in the art. The chemical sensitizers increase the general sensitivity of photographic emulsions in which they are incorporated, that is, the sensitivity of the emulsion is increased to light of substantially the same wave lengths as those to which the emulsion would be sensitive without the incorporation of any sensitizer. The optical sensitizers also may increase the sensitivity of an emulsion in the region in which it was originally sensitive; however, their principal purpose is usually to extend the range of sensitivity, that is, to make the emulsion sensitive to some group of wave lengths to which it was not originally sensitive. The present invention is limited to the use of basic or neutral optical sensitizers and does not extend to the use of chemical sensitizers, such as the thiourea or other sulfur-containing chemical sensitizers. The sulfur-containing chemical sensitizers have no effect in increasing the reversal characteristics of an emulsion, although the sulfur-containing optical sensitizers, do increase the reversal characteristics of an emulsion when used in amounts in excess of the optimum amounts for negative sensitizing.

The accompanying drawing illustrates the ef fect of adding optical sensitizing dyes and photographic developers in suitable amounts to emulsions according to my invention. In Fig. 1 the curve A shows the characteristics of an ordinary unsensitized negative emulsion as represented by H. and D. measurements and the curve B shows the characteristics of a similar emulsion to which an optical sensitizing dye has been added. It can be seen that the emulsion represented by the curve B has higher contrast for the same amount of exposure and does not tend to reverse in the region of over-exposure. Although this property is not exhibited by all emulsions, it does exist in most cases.

As shown in Fig. 2, when an optical sensitizing dye is added to this emulsion in excess of the amount which would be used for optimum negative sensitivity a point is reached at which increased exposure reduces the contrast as shown by the curve C. This is a unique and unexpected result and would not be supposed from a comparison of the curves A and B, which indicate that ordinarily an increase in sensitivity results from adding an optical sensitizer to the emulsion. Curve D indicates that the reversal elfect may be enhanced by adding a small amount of a photographic developer to the emulsion represented by curve C and the maximum density thereby increased without appreciably increasing the minimum density obtainable. This, of course, indicates that the contrast of the emulsion is greatly increased.

The amount of sensitizing dye used according to my invention in order to secure the best results will usually vary within the limits of 1 to 2.5 grams per 1000 grams of silver bromide in the emulsion. The amount of developing agent will vary from 0 to 200 milligrams per 1000- grams of silver bromide. The amount of sensitizing dye will always be in excess of the amount of developing agent and the. ratio between the two will vary from 1 part of developing agent and 5 parts of sensitizing dye down to a complete ab-.

sence of developing agent. The optimum proportion is about 1 part of developing agent and 25 parts of sensitizing dye.

The following specific examples which are illustrative only, indicate several ways of forming an emulsion according to my invention.

Example I A bromo-iodide emulsion is made by mixing the following ingredients, comprising part A and part B, in the following way:

Part A.

' Potassium bromide g 80.0 Potassium iodide g 2. 5 Pecipitation gelatin g 30.0 28% ammonium hydroxide cc 40. 0 H2O cc 750.0

Part B.

Silver nitrate converted into theammonia complex with ammonium hydroxide g 100.0

H2O cc 750. 0

Part A is maintained at 45 C. and part B is added over a period of 40 minutes. After this precipitation the emulsion is heated at 45 C. for one hour, at the end of which time 10 grams of citric acid and grams of active gelatin are stirred in. After 60 minutes washing and heating to maximum sensitivity, the emulsion so prepared shows very little tendency to reverse, the shoulder of the characteristic curve being rather low and flat.

To this emulsion there is added 0.10 gram of 2 [4(1-piperidyl) n butadienyllpl-napthiazol ethiodide. The emulsion is coated and dried and on exposure and development shows good reversal characteristics with a low minimum density and good latitude.

Example II densities.

Example III To the emulsion of Example I are added 0.15 gram of 3-ethyl-5- -(l-piperidyl) allylidene rhodanine and 0.0075 gram of hydroquinone. This emulsion is coated and dried, and shows excellent reversal characteristics.

Example IV To the emulsion of Example I are added 0.125

gram of 3-ethyl-l-phenyl-5l' -(l piperidyl) allylidene]-2-thiohydantoin and 0.005 gram of phydroxylphenylglycine. This emulsion is coated and dried, and gives a good reversal curve.

Example V Emample VII To the emulsion of Example I are added 0.175 gram of 4-methyl-2-p-diethylamino-styryl-thiazole ethiodide and 0.005 gram of pyrogallol. This emulsion on coating and drying gives good reversal.

Example VIII To a bromo-iodide emulsion, made from 100 grams of neutral silver nitrate, are added 0.20 gram of 2,2-diethyl-S-methylthiacarbocyanine iodide.- This emulsion is coated and dried and shows good reversal characteristics.

In the preparation of my emulsions there is no heating step after addition of the sensitizing dye and the developing agent and, therefore, no fog is produced. I may flash the emulsion after coating and drying but no improvement in photographic quality results from this step.- Any advantage would be merely one of securing more uniform results in the hands of difierent operators. The pre-fiashing operation consists of exposing the film after coating and drying, for a sufflciently short time to attain the maximum density without entering the reversal side of the exposure curve, It may be done to facilitate handling the film under safelights giving varying degrees of illumination, and affects principally the edge of the film, without appreciably affecting the main image portion.

I have found that certain developing solutions I give better results than others with my emulsions.

In general, developers having low silver solvent action are better than those showing solvent properties. For example, developers with low sulfite concentrations give lower minimum densities than those containing a largeamount of sulfite. However, there are exceptions to this general rule and it is to be understood that the development of my emulsions is not confined to low sulfite developers. A developer suitable for use with my direct reversal emulsions may have the following formula:

Hydroquinone grams 5 Monomethyl p-aminophenol sulfate gram 1 Sodium sulfite grams 10 Borax do 10 Potassium bromide gram 1 Water to liter 1 The emulsion made according to my invention may be used to produce normal negatives as well as direct positives. In this case the emulsion is not flashed after coating and the exposure must be carefully controlled so that the reversal portion of the characteristic curve is not entered. My invention is susceptible of other variations not specifically described herein and it is to be understood that I wish to be limited'only as defined by the scope of the appended claims.

What I claim is:

1. The method of making a direct positive emulsion, which comprises incorporating in a gelatino-silver halide emulsion a non-acid optical sensitizing dye for the visible region in an amount approximately 1 to 4 times that necessary to give the optimum sensitizing for a normal negative emulsion, and coating the emulsion on a support without fogging it prior to exposure.

2. The method of making a direct positive .emulsion, which comprises incorporating in a gelatino-silver halide emulsion a non-acid optical sensitizing dye for the visible region in an amount approximately greater than that necessary to give the optimum sensitizing for a normal negative emulsion, maintaining the emulsion at a temperature below that at which fog is produced, and coating the emulsion on a support.

3. The method of making a direct positive emulsion, which comprises incorporating in a gelatino-silver halide emulsion a non-acid optical sensitizing dye for the visible region in an amount approximately 1 to 4 times that necessary to give the'optimum sensitizing for a normal negative emulsion, and a photographic, developing agent, and coating the emulsion on a support without fogging it prior to exposure.

4.. The method of making a direct positive emulsion, which comprises incorporating in a gelatino-silver halide emulsion a non-acid optical sensitizing dye for the visible region in an amount approximately 50% greater than that necessary to give the optimum sensitizing for a normal negative emulsion, and a photographic developing agent, maintaining the emulsion at a temperature below that at which fog is produced, and coating the emulsion on a support.

5. The method of making a direct positive emulsion, which comprises incorporating in a gelatino-silver halide emulsion an optical thiocarbocyanine sensitizing dye for the visible region in an amount approximately 1 to 4 times that necessary to give the optimum sensitizing for a normal negative emulsion, and a photographic developing agent, and coating the emulsion on a support without fogging it prior to exposure.

6. The method of making a direct positive emulsion, which comprises incorporating in a gelatino-silver halide emulsion a thiocarbocyanine optical sensitizing dye for the visible region in an amount approximately 1% to 4 timesth'at necessary to give the optimum sensitizing for a normal negative emulsion, and a photographic developing agent, the sensitizing dye and the developing agent being present in the ratio of approximately 25 to 1, and coating the emulsion on a support without fogging it prior to exposure.

7. The method of making a direct positive emulsion, which comprises incorporating in a gelatino-silver halide emulsion containing the equivalent of 1000 grams of silver nitrate, approximately 1.25 grams of 2:2-dimethyl-8-ethylthiocarbocyanine iodide and 0.05 gram of a pphenylenediamine developing agent, and coating the emulsion on a support without fogging it prior to exposure.

8. The method of making a direct positive emulsion, which comprises incorporating in a gelatino-silver halide emulsion a merocyanine optical sensitizing dye for the visible region in an amount approximately 1 /2 to 4 times that necessary to give the optimum sensitizing for a normal negative emulsion, and a photographic developing agent, and coating the emulsion on a support without fogging it prior to exposure.

9. The method of making a direct positive emulsion, which comprises incorporating in a gelatino-silver halide emulsion a merocyanine optical sensitizing dye for the visible region in an amount approximately 1% to 4 times that necessary to give the optimum sensitizing for a normal negative emulsion, and a photographic developing agent, the amoimt of sensitizing dye being greatly in excess 01 the amount of developing agent, and coating the emulsion on a support without fogging it prior to exposure.

10. The method of making a direct positive emulsion, which comprises incorporating in a gelatino-silver halide emulsion containing the equivalent of 1000 grams oi. silver nitrate, approximately 1.25 grams of 5(2-ethyl-l-benzoxazlidene-ethylidene) -3-ethyl-rhodanine and 0.10

gram of monomethyl p-aminophenol sulfate, and coating the emulsion on a support without fogging it prior to exposure.

11. The method of making a direct positive emulsion, which comprises incorporating in .a-

optical sensitizing dye for the visible region in an amount approximately 1 to 4 timesthat necessary to give the optimum sensitizing for a normal negative emulsion, and a, photographic developing agent, the amount of sensitizing dye being greatly in excess of the amount of developing agent, and coating the emulsion on a support without fogging it prior to exposure.

13. The method of making a direct positive emulsion, which comprises incorporating in a gelatino-silver halide emulsion containing the equivalent of 1000 grams of silver nitrate, approximately 1.0 gram of 2[4(1-piperidyl-)A butadienyllpnaphthiazol ethiodide and 0.05 gram of a p-phenylenediamine developing agent, and coating the emulsion on a support without fogging it prior to exposure.

14. In the method of making a direct positive emulsion which comprises incorporating in a gelatino-silver halide emulsion a nonacid optical sensitizing dye for the visible region in an amount approximately 50% greater than that necessary to give the optimum sensitizing for a normal negative emulsion, maintaining the emulsion at a temperature below that at which ,fog is produced, and coating the emulsion on a support, the step of pre-flashing the emulsion after coating.

JOHN A. LEERMAKERS. 

